1. Field of the Invention
The present invention relates to a method of detecting battery degradation level, and in particular to a method of detecting degradation level optimally suited for a battery used in a power source apparatus that drives an electric-powered vehicle or a battery used in a solar cell power storage unit.
2. Description of the Related Art
A rechargeable battery degrades with charge-discharge cycle repetitions and the capacity to which it can be charged decreases. Since degradation increases due to over-charging and over-discharging, degradation can be reduced and battery lifetime extended by preventing over-charging and over-discharging, and by limiting charging and discharging operation in regions close to over-charging and over-discharging. To implement this type of operation and achieve reduced degradation and increased lifetime in a battery that drives a vehicle such as a hybrid vehicle (HV, hybrid car, hybrid electric vehicle, HEV), the battery is charged and discharged within a specified range of residual capacity (state-of-charge, SOC [%]). For example, battery charging and discharging can be controlled within a residual capacity (SOC [%]) range of 50%±20%. To control charging and discharging and keep the residual capacity (SOC [%]) within a specified range, it is necessary to detect the full-charge capacity. This is because residual capacity (SOC [%]) is represented by the ratio of the amount of charge (Ah) capable of being discharged to the full charge capacity (Ah). The full-charge capacity can be determined by integrating the discharge current when the battery is discharged from a fully-charged state to a completely discharged state. Or, the full-charge capacity can be determined by integrating the charging current required to fully-charge the battery from a completely discharged state. However, full-charge capacity determination by these methods requires the battery either to be charged from a fully discharged state to a fully-charged state or to be discharged from a fully-charged state to a completely discharged state. For a battery used in an operating environment that does not completely discharge or fully-charge the battery, full-charge capacity cannot be detected by simply integrating the discharge current or charging current. For example, in a hybrid vehicle application, the vehicle cannot be driven by the battery if it is completely discharged, and the battery cannot be charged by regenerative braking if it is fully-charged. Further, for a battery charged by solar cells, solar cell output depends on weather conditions and battery discharge depends on load requirements. Consequently, it can be difficult to operate in a fully-charged state or completely discharged state in a solar cell power storage application as well.
Since full-charge capacity cannot be determined by integrating current in applications that limit battery degradation by controlling charging and discharging within a given residual capacity (SOC [%]) range, an alternate method of determining full-charge capacity is required. Charging and discharging cannot be controlled within a specified range of residual capacity (SOC [%]) if the full-charge capacity is not accurately detected. Consequently, accurate detection of the full-charge capacity is necessary for this type of limited charge-capacity-range operation. Since batteries have the property that full-charge capacity changes as battery degradation proceeds, full-charge capacity can be detected by accurately detecting the degree of battery degradation. Consequently, accurate determination of the level of battery degradation is particularly important for a battery that has its charging and discharging controlled within a given residual capacity range.
A method of integrating battery charging and discharging current has been developed to detect battery degradation level.
Refer to Japanese Laid-Open Patent Publication 2008-228492
According to the rechargeable battery charging method cited in JP 2008-228492 A, battery charging current is integrated and saved in a cumulative manner, each time the cumulative integrated value becomes equal to the full-charge capacity of the battery, it is counted as one cycle, and the battery degradation level is determined as the cumulative cycle count increases. This method detects battery degradation level from the integrated charging current. Specifically, the battery is assumed to degrade as the integrated charging current increases, and that relation is used to detect the degradation level.
Although the disclosed method detects degradation level from the integrated current, battery degradation level is not determined exclusively by the integrated current value, and different degradation levels can result even when the integrated current value is the same. For the case where the integrated current values are the same, degradation level is larger for varying current conditions than for constant current conditions. For example, when 10 A of current flows for 10 sec, the integrated current value is 100 Asec, and when a current peak of 100 A flows for 1 sec while no current flows for the remaining 9 sec, the integrated current value is also 100 Asec. However, the battery degradation level for a 100 A peak current flow is greater than that for constant 10 A current flow conditions. As a result, a method that detects degradation level based only on integrated current values has the drawback that an accurate battery degradation level cannot always be determined.
The present invention was developed to further resolve the drawback described above. Thus, it is a primary object of the present invention to provide a method of detecting battery degradation level that can more accurately detect the degradation level from the battery current.